Shop Windows to the Universe

These two photographs of a flower show how the evening primrose looks in two different wavelengths. The upper panel shows the flower as humans see it in visible light. The lower panel shows the primrose in UV, and reveals the "honey guides" (the dark areas) invisible to the human eye but seen by insects. The dark lines and patches guide the insect to the collection of nectar stored in the center and to the pollen on the anthers.Image courtesy Dr. Jeremy Burgess, Science Source/Photo Researchers.

The ultraviolet spectrum is sometimes subdivided into the near UV (380
to 200 nanometer wavelengths) and extreme UV (200 to 10 nm wavelengths). Normal
air is largely opaque to UV with wavelengths shorter than 200 nm (the extreme
UV range); oxygen absorbs "light" in that part of the UV spectrum.

In terms
of impact on the environment and human health (and choosing sunglasses!),
it can
be useful
to subdivide
the
UV spectrum in a
different
way, into UV-A
("blacklight"
or
Long
Wave UV with a 380 to 315 nm wavelength), UV-B (Medium Wave at 315 to 280
nm), and UV-C (the "germicidal" or Short Wave UV that ranges from
280 to 10 nm).

Earth's atmosphere prevents most UV
radiation from space from reaching the ground. UV-C is entirely screened out
by stratospheric ozone at around 35 km altitude. Most UV-A does reach the
surface, but UV-A does little
genetic damage to tissues. UV-B is largely responsible for sunburn and skin
cancer, though it is mostly absorbed by ozone before reaching the surface.
Levels of UV-B radiation at the surface are especially sensitive to levels
of ozone in the stratosphere.

Ultraviolet radiation causes sunburn. It is used to
sterilize glassware used
in medicine and biological research.

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